This work was supported by the Ministerio de Ciencia e Innovación (MICINN; projects CTQ2006-01121 and CTQ2009-07791) and the Consejería de Educación de la Comunidad de Madrid (programme AVANCAT; S2009/PPQ-1634). A.G.-R. and B.U. thank the MICINN for a predoctoral fellowship. We also thank Johnson Matthey PLC for a generous supply of Pd(OAc)2.